1. Field of the Invention
This invention relates to cellular communications and, in particular, to a system that enables wireless subscriber stations that are located in an aircraft to receive consistent wireless communication services in both the terrestrial (ground-based) and non-terrestrial (airborne) regions via the management of call handoffs among cells as the aircraft traverses the coverage area of these cells.
2. Problem
It is a problem in the field of wireless communications to efficiently and continuously meet the communications requirements of multiple aircraft as the aircraft fly their routes and approach/depart airports.
A typical air-to-ground cellular communications network consists of a number of ground stations, each of which provides a radio frequency coverage area in a predetermined volume of space radially arranged around the cell site transmitting antenna, using an antenna pattern which is insensitive to the reception of ground-originating or ground reflected signals and which antenna pattern is transmissive only in a skyward direction. The ground stations are geographically distributed, generally following a cellular network layout. The coverage area of each ground station is substantially contiguous with that of neighboring sites, so that the composite coverage of all of the ground stations in the cellular communications network generally provides coverage over the targeted area. Ground stations may provide either a single cell of coverage using transceiver(s) associated with a single transmit and receive antenna system or multiple cells of coverage, each with associated transceivers and transmit and receive antennas. The advantage of the latter arrangement, with multiple cells per ground station is to allow provision of increased call handling capacity in the coverage area of that ground station.
There are limitations on the total radio frequency spectrum available, and therefore limitations on the total available call handling capacity in any single cell. As a wireless communications device moves from the coverage area of one cell into the coverage area of a contiguous cell, the communications from that wireless communications device are handed over from the first cell to the second cell. This requires that there be adequate available call handling capacity in the second cell to support the new load represented by this wireless communications device. Call handoffs that entail a single wireless communications device do not create large transient loads on a cell. However, in an air-to-ground system, the number of presently active cellular calls within an aircraft may represent a large fraction of the call handling capacity of a terrestrial air-to-ground cell. In addition, aircraft arrivals and departures from the coverage area of a cell are infrequent. With the small number of aircraft that can be served by a cell and long average transit times for aircraft within a cell, there must be a large allowance of idle capacity to serve aircraft which may arrive before the presently served aircraft leave the cell and free up call handling capacity within the cell.
There is therefore a need for an improved method of managing the air-to-ground cellular communications from aircraft in order to provide increased communications capacity while meeting service objectives. The management of call handoffs among cells as the aircraft traverses the coverage area of these cells is critical to meeting the communications requirements of multiple aircraft as the aircraft fly their routes and approach/depart airports.